Three groups of semiconductor circuit elements are known for which the lifetime of the minority carriers in the base region must be lowered in a defined fashion in order to achieve a rapid switching characteristic with low switching loss. These groups are:
All three building blocks have in common that they can have two switching states: a conductive state in which the high ohmic base region is flooded with minority carriers, and a blocked state in which the base regions are to a great extent free of charge carriers, so that the blocking voltage can be accommodated.
The transition from the conductive state into the blocked state takes the more rapidly, the more rapidly the charge carriers in the base region recombine. If this recombination process takes too long, current still flows in the blocked phase and considerable switching losses occur which can lead to the destruction of the element.
In general, the lifetime of the minority charge carriers in silicon elements having rapid switching characteristics is adjusted by means of an infusion of a heavy metal, for example gold or platinum, at temperatures between 800.degree. and 1300.degree. C. (See, for example, U.S. Pat. No. 3,067,485). The diffusion of gold into the silicon lattice takes place as a coupled mechanism (Frank Turnbull mechanism): first a rapid intersticial diffusion takes place and later a slow change from the intersticial locations to lattice points, the latter being coupled with a slow infusion of double voids from the surface into the volume. Only the gold atoms situated on lattice points can serve as recombination centers for the charge carrier pairs. The number of recombination centers manufactured by means of gold diffusion depends on the following conditions:
Following the diffusion of the gold, the silicon must be cooled as rapidly as possible to prevent the gold atoms from being recaptured in the highly doped surface zones.
The disadvantage of this method is that the concentration of the gold atoms at the lattice points does not vary only as a function of the diffusion parameters of temperature and time, but also as a function of the characteristics of the crystal and of the cooling conditions.
For these reasons it is difficult to produce a reproduceable number of recombination centers in semiconductor elements by means of gold diffusion.